It has been projected that the potential energy savings from energy-efficient solid-state lighting (SSL), i.e., lighting from solid state sources such as light emitting diodes (LEDs), can be very significant: by 2030, the cumulative energy savings due to SSL is estimated to be approximately 1,488 TeraWatt-hours ($120 billion at today's energy prices) while reducing greenhouse gas emission by 246 million metric tons of carbon.
The organic analog of the well-known inorganic LEDs, referred to as OLEDs, are being used to produce ultra-thin and/or curved/bendable televisions, displays, cameras, mobile devices, etc. OLEDs are also being developed for unique applications in lighting, including general, decorative and architectural lighting, signage and advertising, backlighting for displays, automotive and aircraft lighting, etc. OLEDs can be fabricated with a wide range of characteristics to produce unique light sources that are ultrathin, flexible, lightweight, and can cover large areas, etc. OLEDs have the potential to provide a new form of lighting with novel and unique surfaces and shapes.
While the advantages and benefits of OLED lighting are numerous, few such devices are currently available. OLED devices produced by the current technology suffer from a number of limitations, including low light output efficiency, poor light uniformity over large areas, and very critically, very high cost. These limitations inhibit the wide-scale adoption of these devices and the realization of their potential commercial and environmental benefits. Although a number of approaches have been proposed for overcoming these limitations, they are difficult and costly to implement, and often offer minimal performance improvements. The systems and methods disclosed herein overcome a number of key limitations of the prior art.
One major challenge for OLEDs is poor light-extraction efficiency, because much of the light produced in the organic layers is lost due to waveguide trapping due to refractive index mismatching within the multi-layer OLED device stack (“internal light trapping”), and additional light is lost by total internal reflection (TIR) at the substrate/air interface (“external light trapping”). Thus a significant amount of the light generated within an OLED device stack is trapped and lost. Prior art methods for extracting this light, which include use of buckled or undulating surfaces for internal light extraction and microlens arrays and other relief structures for external light extraction, add a high degree of complexity and cost to the manufacturing process, contributing to limited commercial viability for mass-market applications.
Another critical area for OLED devices, as well as to other emissive and light absorbing (photovoltaic and the like) devices, is the transparent electrical conductive (‘TC’) layer. The most widely used TC materials are transparent conductive oxides, or ‘TCO’s, including indium tin oxide (‘ITO’) and related oxides, have a number of drawbacks, including insufficient electrical conductivity, poor optical transmissivity, brittleness, high surface roughness, and tendency for adhesion failure, all of which are exacerbated when TCO's are deposited on flexible plastic substrates. Other transparent conductive materials commonly in current use include organic polymers such as poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS), ultra-thin metal layers, coatings of silver and other nanoparticles (nanotubes, nanowires, nanobuds, etc.), graphene layers, etc. However, as transparent conductor layers these materials all suffer from one or more drawbacks, in particular inadequate electrical conductivity and/or poor optical transmission. Performance tradeoffs typically require sacrifice of optical transmission to achieve higher conductivity, and vice versa. Because OLEDs are current-driven devices, the high resistance of these conductive layers and their inability to handle significant electrical current flux results in emissivity drop off with increasing electrode distance, producing non-uniform light output.
In order to provide adequate electrical conductivity, TCOs such as ITO must be coated very thickly, which in addition to reduced transmission also results in high surface roughness and a tendency to crack and even delaminate from flexible substrates. The resulting surface irregularities produce severe thickness variations in the device stack, such as thinning at sharp external corners of projecting surface structures and pooling at internal corners of depressions, etc. Because OLEDs are extremely sensitive to surface roughness and sharp non-uniformities, these defects are significant sources of poor device performance and device failure.
Another type of transparent conductive layer is the metal grid or mesh, which can offer potentially very high electrical conductivity and optical transmission. Metallic wire grids can be formed by graphic arts printing (ink jet, gravure, flexo, etc.), by electro- or electroless plating, or by vacuum deposition. These techniques produce lines with significant thickness compared to the thickness of the OLED layers coated on them, especially for metallic lines commonly made by printing. In theory, these steps can be eliminated by planarization using typical dielectric planarizing materials, such as polymers, silicon oxides, etc., but this is a problematic process that can easily result in overcoating and thereby insulation the lines. Chemical mechanical polishing (CMP) can be used to planarize such surfaces, but it is not practical for large area devices or for R2R manufacturing. An additional problem with wire grid transparent conductors is the lack of electrical conductivity in the areas between the metal wires. This is not a problem for some grid applications, such as touch screens, but the lack of continuous electrical conductivity over the entire transparent electrode surface does present a problem for OLEDs and other devices (photovoltaic PV, etc.)
As mentioned, one of the most effective extraction structures is formed by periodic or quasi-periodic undulating surfaces, as has been described in recent scientific literature. This approach to light extraction produces other problems, in particular when attempting to combine such an internal light extraction structure with a metal grid. In order to produce best extraction performance from such relief structures, the conductive layer is preferably conformal with the undulating structure—that is, following the contours of the underlying structure. For example, simply combining a (planar) grid with an undulating surface formed over the grid will result in insulation of the grid, since (insulating) polymeric layers are the only viable method for forming such structures. Thus, a means for providing an internal light extraction structure with a high conductivity conformal metal grid, while simultaneously providing a smooth surface for device deposition, and in particular at a commercially viable cost, is currently lacking.
Another important issue for OLEDs and many other organic electronic devices (TFT's, etc.) is extreme sensitivity to moisture and oxygen, which can result in premature device failure for even low levels of such exposure. OLED devices must be carefully protected from these elements, typically through the use of barrier layers and encapsulation sealants, often done by placing the device between glass or glass/metal plates and using epoxy or other oxygen/moisture impermeable sealants to greatly reduce or eliminate ingress of moisture and oxygen. The anode/cathode electrical device connections must also be made through this barrier, further complicating the structure and manufacturing process.
Numerous solutions have been proposed to overcome the described multiple limitations of the prior art, but, as mentioned, all add significantly to the complexity and cost of manufacturing. What is needed is a cost-effective means of incorporating various improvement technologies into these devices.